This invention relates to semiconductor memory devices, and more particularly to MOS read only memories and a process for making them.
Semiconductor memory devices are widely used in the manufacture of digital equipment such as minicomputers and microprocessor systems. Storage of fixed programs is usually provided in these systems by MOS read only memory devices or "ROMs". ROMs are made by semiconductor manufacturers on special order, the programming code being specified by the customer. The manufacturing process is lengthy, requiring dozens of steps, each taking up time and introducing materials handling and inventory factors. Customers require the turn-around time or cycle time between receipt of the ROM code for a custom order and delivery of finished parts to be kept as short as possible. For this reason, programming should be done late in the manufacturing process, but previous ways of doing this required large cell size. The economics of manufacture of ROMs, and of mounting them on circuit boards in the system, are such that the number of memory bits per semiconductor chip is advantageously as high as possible. ROMs of up to 32K bits (32,768) are typical at present. Within a few years, standard sizes will progress through 64K, 128K, 256K and 1 megabit. This dictates that cell size for the storage cells of the ROM be quite small. Metal gate ROMs of small size can be relatively easily fabricated in the manner set forth in U.S. Pat. No. 3,541,543, assigned to Texas Instruments, but usually these are programmed by the gate level mask which is at an early stage in the process. A method of programming a ROM by ion implant prior to forming the polysilicon gate is shown in U.S. Pat. No. 4,059,826 to Gerald D. Rogers, assigned to Texas Instruments. Also, previous cells have been programmed at the metal level mask by contact areas between metal lines and polysilicon gates, or by contacts between metal lines and N+ source or drain regions, using excessive space on the chip. And more recently post-metal programmable ROMs have been disclosed in pending applications, Ser. No. 890,555, filed Mar. 20, 1978 by C-K Kuo, Ser. No. 912,635, filed June 15, 1978 by Chatterjee and Tasch, and Ser. No. 912,774, filed June 5, 1978 by Chatterjee and Tasch, all assigned to Texas Instruments.
It is the principal object of this invention to provide a method for programming MOS ROMs after the completion of the slice manufacturing process.